Nobody is perfect, not even proofreaders. But first, let's get some terminology straight:
Copyediting is a catch-all term for editors who revise, make changes and suggestions, and so on. Of course, as editors go along, we mark up any typos they find on the proofreading level. But it's not the focus of this pass. We'll catch as many errors as we can, but at this point, we're looking to make the work read better while preserving the writer's voice.
Proofreading is done after copyediting, with the intent of catching all errors that absolutely cannot make it into print. It should be a separate pass from editing, Since a writer has to accept or reject the editor's changes, this process can actually introduce typos. This is, of course, despite the fact that editors never make mistakes.
In the world of self-publishing and small presses, these two steps are often combined. This results in more typos.
The dirty little secret of the editing world is that, in any work that's long enough, despite our very best efforts, there will be typos that weren't caught. There will be places where someone made a last-minute change in the post-galley stage, or a proofreader simply didn't catch something. This is why writers with a large following often enlist the aid of readers to catch typos, often many of them at the same time. This is also the case with writers who are working with large publishing houses. I've even seen writers who will specifically ask readers who bought the hardcover to email in any typos they found.
So, if it's important to you that your work be as close to error-free as possible, I suggest always checking a proofreader's work. It's not insulting or a slight on their professionalism. Enlist the help of some friends who will go on a typo hunt. If you have the budget, hire a second proofreader.
Even if all you do is catch one or two typos, multiple layers of editing and proofreading is a big help and makes your book look more professional.